Polyaniline has emerged as one of the more promising conducting polymers, because of its excellent chemical stability combined with respectable levels of electrical conductivity of the doped or protonated material. Processing of polyaniline into useful objects and devices, however, has been problematic. Melt processing is not possible, since the polymer decomposes at temperatures below a softening or melting point. In addition, major difficulties have been encountered in attempts to dissolve the material.
Conflicting reports exist on the solubility of polyaniline, with contradictory views advanced as early as 1910. Willstatter and Dorogi reported that an oligomeric (eight-monomer chain compound) aniline was largely insoluble. Willstatter et al. (1909) Ber 42:2147; id. at 4118. Green and Woodhead repeated their experiments and claimed solubility of this nonpolymeric material in 80% acetic acid, 60% formic acid, pyridine and concentrated sulfuric acid. Green et al. (1910) J. Chem. Soc. 97:2388; Green et al. (1912) J. Chem. Soc. 101:1117. In recent years, Angelopoulos and coworkers and Wang et al. reported only partial solubility of polyaniline, in its emeraldine base form, in N-methylpyrrolidone (NMP), dimethylformamide (DMF), tetrahydrofuran (THF), benzene and chloroform. Angelopoulos et al. (1987) Synth. Met. 21:21); Wang et al. (1986) Synth. Met. 16:99. As a result, it has become common, laborious practice to "remove insoluble material", and use the soluble, probably oligomeric, polyaniline fraction for the preparation of films. Angelopoulos et al. (1987) Synth. Met. 21:21. Such cast films are predominantly amorphous, much like the as-synthesized material. Annis et al. (1986) Synth. Met. 22:191; Andreatta et al. (to be published). More recently, Watanabe and coworkers claimed total insolubility of the emeraldine salt of polyaniline in any solvent. Watanabe et al. (1987) Chem. Commun. 3.
Trevoy, U.S. Pat. No. 4,025,342, reports that emeraldine sulfate is an insoluble microcrystalline powder, thus lacking utility in practical conducting coatings. Trevoy proposes to solve this problem by combining short oligomers of aniline salts with other, more easily processed polymers. Jasne, U.S. Pat. No. 4,731,408, also recognized the need for improved processability of conducting polymers such as polyaniline, noting particularly insolubility as a problem. Jasne proposes preparing polyaniline by polymerizing the monomer in a latex dispersion which serves as a counter ion, the latex comprising 50-97% by weight of the resulting film. Yang et al., U.S. Pat. No. 4,586,792, also recognizes the insolubility of polyaniline.
Routes towards soluble polyaniline include the preparation of graft and co-polymers, and polyaniline derivatives. Li et al. (1987) Synth. Met. 20:141; Li et al. (1988) Synth. Met. (in press); Wang et al. (1986) Synth. Met. 16:99; Ray et al. (19880 Synth. Met. (in press). Unfortunately, these species invariably show significantly reduced conductivities in comparison with the (unmodified) homopolymer.
Until now, this polyanaline generally has been categorized as intractable and amorphous. Polyaniline articles, therefore, are typically fabricated through elaborate compaction techniques, yielding relatively poor mechanical coherence, frequently with poly(tetrafluoroethylene) binder material. Thus, the ability to fabricate high quality, crystalline polyaniline into shaped articles such as fibers, films and the like remains seriously limited, despite nearly eighty years of research.